String formatting: % vs. .format vs. string literal

Question

Python 2.6 introduced the str.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?

Pyt

Answer 1

One situation where % may help is when you are formatting regex expressions. For example,

'{type_names} [a-z]{2}'.format(type_names='triangle|square')

raises IndexError. In this situation, you can use:

'%(type_names)s [a-z]{2}' % {'type_names': 'triangle|square'}

This avoids writing the regex as '{type_names} [a-z]{{2}}'. This can be useful when you have two regexes, where one is used alone without format, but the concatenation of both is formatted.